The present invention relates to a process and apparatus for the casting of fused refractory oxide materials having high fusion points, such as magnesia. Such refractory oxide materials have melting points greater than about 2400.degree. C. Many refractory oxide materials, such as alumina, have relatively low melting points and are commonly commercially produced on a continuous or semi-continuous basis by pouring or tapping the molten oxide from a furnace. However, such methods have not been successfully employed with refractory oxides having melting points over about 2400.degree. C. because the fluidity of such material decreases at a very rapid rate with a small decrease in temperature, causing the material to solidify in the furnace or to a great extent during pouring before any significant amount of product can be poured or drawn from a tap.
The present method and apparatus are particularly adapted to the production of fused magnesia (MgO). The melting point of magnesia is about 2800.degree. C. Fused magnesia is typically commerically produced by batch processes using electric arc furnaces of the open arc type from feed material of calcined magnesia or magnesite having a high MgO content.
Conventionally fused magnesia is produced by placing a metal container, suitably a steel shell, partially filled with a bed of feed material, such as calcined magnesia, under a set of graphite or carbon electrodes. A bridge of carbon or graphite, suitably in the form of resistor bars, is positioned in the bed to form an electrical connection, or bridge, between the electrodes when they are lowered. Current passing through the bridge causes local heating and subsequently the formation of an initial molten oxide pool. When the pool is of a size sufficient to conduct electric current by itself, the electrodes are moved to a position sufficiently close to the surface of the pool so that electric current passes through the arc to the molten pool. The container is filled by gradually feeding additional feed material in the area of the molten bath. As the molten area increases in size, a surrounding layer of feed material congeals to form a container for the fused magnesia. This layer is generally termed the "skull". Between the skull and the sides of the container, a second layer of partly solidified material is formed, which, in turn, is surrounded by a layer of substantially unaffected feed material. The skull layer and the surrounding layers of unfused feed material serve to insulate the shell from the high temperatures of the bath. To further cool the shell, water is sprayed on the outside portions during the fusion operation and, if desired, during the cooling stage.
Typically, fused magnesia is formed, in situ, by allowing the contents of the furnace to cool sufficiently to solidify the bath. The fused magnesia product, usually in the form of a large mass, generally referred to as a "pig", is removed from the furnace after cooling by chipping with pnuematic hammers or, more frequently, by dumping the furnace contents on a sorting floor and separating the product from the congealed material and the unfused material. The congealed material and the unfused material are recovered and utilized as feed material in subsequent fusions. The pig is broken into smaller pieces as a feed material for a crusher or a subsequent milling operation. The product, granules or grains of fused magnesia, is useful in the production of shaped refractory materials by being bonded together, with or without additional refractory materials, shaped and fired. The particulate product is also useful as an electrical insulator material in tubular heating elements, such as those used in electric stoves, water heaters, cooking pots and frying pans.
The working of the pig, i.e., separating and breaking the pig to obtain pieces suitable for crushing, is time-consuming and requires a substantial amount of arduous labor.
The present invention provides a method and apparatus for the semi-continuous production of a cast, fused magnesia product. The term "semi-continuous" means that more than one run, or pour, of fused magnesia may be produced from a furnace without removal of the skull from the furnace. The cast product may be produced in a thin layer having a relatively smooth surface, particularly adapted to being easily fractured into pieces suited to crushing and requires no separation of fused from unfused material.